1. Field of the Invention
The invention relates to the purification of gases in a pressure swing adsorption system. More particularly, it relates to the enhancing of product recovery and/or purity in such systems.
2. Description of the Prior Art
The pressure swing adsorption (PSA) process and system provide a well established means for separating and purifying at least one gas component from a feed gas mixture of said component and at least one selectively adsorbable component. The process includes adsorption of said selectively adsorbable component at a higher adsorption pressure, and pressure reduction to a lower desorption pressure to desorb said selectively adsorbable component. It is generally desirable to employ the PSA process in multi-bed systems such as those described in the Wagner patent, U.S. Pat. No. 3,430,418, in which at least four adsorption beds are employed. The PSA process is carried out in such systems, on a cyclic basis, employing a processing sequence that includes (1) high pressure adsorption, (2) cocurrent depressurization to intermediate pressure levels, with release of void space gas from the product end of the bed, (3) countercurrent depressurization or blowdown to a lower desorption pressure bed, (4) repressurization to the higher adsorption pressure. Wagner teaches the passing of a portion of the void space gas from one bed during cocurrent depressurization directly to another bed initially at its lower desorption pressure so as to equalize the pressure in the two beds at an intermediate pressure level. In a further development, the Fuderer et al patent, U.S. Pat. No. 3,986,849, teaches the use of three such cocurrent depressurization-direct pressure equalization steps in adsorption systems having at least seven beds, with two of the beds being on their adsorption step, in overlapping processing sequence, at all stages of the processing operation.
The use of cocurrent depressurization-direct pressure equalization steps, as is taught by the patents referred to above, enables highly efficient processing cycles to be practiced in hydrogen purification and other practical PSA applications. Multi-bed systems employing such cycles combine enhanced productivity with a substantially uniform flow of product effluent therefrom. While such direct pressure equalization systems are highly advantageous for many applications, known competitive PSA cycles can be based upon the use of cocurrent depressurization steps with the release gas being passed to external vessels for indirect pressure equalization purposes. Thus, systems can be employed in which no direct pressure equalization steps are used. A quite efficient indirect cycle known in the art employs a series of indirect equalizations into external vessels provided with heat capacity packings to reduce temperature swing effects and to increase effective gas storage in the external vessels.
A disadvantage of indirect pressure equalizations is the cost of the gas storing vessel or vessels employed, although this is partly compensated for by the reduced number of valves, and the very simple piping, pertaining in such indirect pressure equalization systems. For instance, a system employing six indirect pressure equalizations can be used to achieve a product recovery essentially the same as an eight or ten bed system employing three direct pressure equalization steps. Because of the larger number of valves and controls, and the more complex piping involved, desirable systems based on direct pressure equalizations, on the other hand, become increasingly costly as the number of direct equalization steps is increased. Each such direct equalization generally requires an additional valve for each bed, e.g. ten more valves for each additional direct equalization in a ten bed system.
Thus, both direct pressure equalization systems and indirect pressure equalization systems have advantages and limitations in commercial practice. The direct pressure equalization systems have the lowest vessel cost, but the highest valve and control costs, whereas indirect systems have the highest vessel costs, but the lowest valve and control costs. It is desirable in the art to develop processes and systems for providing the advantages of direct pressure equalizations at lower overall costs. It is likewise desirable to improve the efficiency of indirect equalizations without undue increases in the overall costs of such processes and systems, particularly as applied to greater flowrate operations. As those skilled in the art will also appreciate, it is always further desired to enhance product recovery and/or purity in practical commercial pressure swing adsorption operations.
It is an object of the invention, therefore, to provide an improved pressure swing adsorption process and system.
It is another object of the invention to provide a PSA process and system enhancing product recovery and/or purity.
It is a further object of the invention to provide a PSA process and system having a desirable balance of enhanced efficiency and reduced costs for valves and related controls.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.